There is an important unmet need to identify small-molecules that can target breast cancer stem cells (CSCs). CSCs are responsible for driving tumor growth and metastasis. CSCs are also known to be resistant to most conventional therapies, including chemo- and radiation-therapy, clearly underscoring the need for therapies that target CSCs. The objective of the proposed study is to identify small-molecule probes that are selectively toxic to breast CSCs. Such molecules would be immensely useful to probe the biology of CSCs and would also serve as promising therapeutic drug candidates. We propose to screen for small molecules that target breast CSCs using a novel method we have developed that takes advantage of a recently discovered connection between breast CSCs and epithelial-mesenchymal transdifferentiation. In preliminary experiments, we have conducted a proof-of-principle pilot high-throughput screen that successfully led to the identification of a novel compound with specific toxicity for breast CSCs. The screen assay is robust, sensitive, and straightforward. Importantly, the secondary assay for the screen uses a cell-line reagent that is essentially isogenic with the cell line-reagent used from the primary screening assay. This minimizes the likelihood of find spurious compound hits unrelated to the process of interest, thereby providing a very useful advantage not present in most screens. The success of the pilot screen gives a very high degree of confidence that the proposed screen, if funded, would provide useful reagents for basic research and potential therapies targeted breast CSCs. PUBLIC HEALTH RELEVANCE: Breast cancer stem cells are responsible for driving tumor growth and are currently poorly understood. Breast cancer stem cells are also resistant to current anti-cancer treatments, indicating that they are major barriers to effective treatment. We propose to collaborate with the MLPCN to employ a novel, validated high-throughput cell-based screen to identify selective inhibitors of breast cancer stem cells.